1. Field of the Invention
This invention relates generally to clamping systems and methods and more particularly to claiming systems and methods for releasably securing a printing plate to a plate cylinder.
2. Description of the Related Art
In offset lithography, an image is present on a printing plate as a pattern or xe2x80x9cimagexe2x80x9d of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a typical sheet-fed offset press system, the imaged plate is mounted to a plate cylinder, where it is inked. The plate is then brought into contact with the compliant surface of a blanket cylinder. The blanket cylinder, in turn, applies the image to paper sheets which are brought into contact with the blanket cylinder by an impression cylinder.
Although the plates for an offset press were traditionally imaged photographically, more recently, a number of electronic alternatives have been developed for placing the image onto the plate. These digitally controlled imaging devices include lasers that chemically alter or destroy one or more plate layers, ink jets that directly deposit ink-repellent or ink-accepting spots on a plate blank and spark or ion discharge devices which physically alter the topology of the plate blank. These various methods of imaging lithographic plates are described in detail in U.S. Pat. Nos. 3,506,779, 4,054,094, 4,347,785, 4,911,075 and, 5,385,092 among others.
Plates can be imaged on-press or, more traditionally, on an off-press platesetter. A digitally operated platesetter includes an imaging cylinder to which the plate is initially mounted, and which carries the plate past the head of the imaging device. That device transfers the image to the plate. The imaged plate is then removed from the platesetter and transferred to the plate cylinder of the printing press.
When mounting an imaged plate to a plate cylinder for a press run or when mounting a plate blank to an imaging cylinder for imaging, it is essential that the leading and trailing edges of the plate be secured firmly to the cylinder and that the plate be wrapped tightly around the cylinder. This ensures that there will be no relative movement between the plate and the cylinder when the cylinder is rotated. Likewise, when a donor/acceptor sheet set is mounted to a cylinder for platemaking by thermal transfer, both sheets must be firmly clamped to the plate to avoid relative movement.
Various devices, including vacuum clamps and mechanical and electromechanical clamps, have been developed over the years for holding a lithographic plate to a plate cylinder. For the most part, theses devices have all tended to be relatively complex and costly. Such devices typically require relatively large and heavy metal plates as components of the clamping mechanism which, when attached to the plate cylinder, create a substantial xe2x80x9cvoidxe2x80x9d segment on the cylinder. Also, in most cases, the clamping mechanisms are fixed to the cylinders such that the mechanisms can only secure a printing plate having a specific length. Since the plate blanks are often pre-cut to fit the specific plate cylinder of the printing press, a separate imaging cylinder, having the same dimensions as the printing cylinder, is generally used to image the plates associated with each printing press. The inability of platesetter and printing cylinders to accommodate differently sized plates substantially increases the cost of operating the printing press.

The present invention enables rapid, efficient mounting of a recording member, such as a lithographic plate to a plate cylinder for printing. It is equally applicable for securing a plate blank or a donor/acceptor sheet for plate-making.
Briefly, the invention utilizes one or more retention devices each having a plurality of discrete magnetic elements separated by interstitial spaces. The retention devices are removed from and disposed onto a cylinder-borne plate by an application/removal system configured to engage the retention devices. When disposed on the plate, the retention devices magnetically adhere the plate to the underlying cylinder.
It is an object of the present invention to provide rapid, efficient mounting of a recording member such as a lithographic plate to a cylinder for printing. It is equally advantageous for mounting a plate blank or a donor/acceptor sheet set to a cylinder for plate-making.
It is a further object of the present invention to provide a plate clamping system to easily accommodate plates of varying sizes.
Accordingly, in a first aspect, an apparatus for securing a recording medium to a rotatable supporting means in accordance with the invention comprises an elongated retention device for releasable attachment to the supporting means over the recording medium, and comprising at least one magnetic element; and an application/removal device for holding and dispensing the retention device. The application/removal device is positionable with respect to the supporting means to facilitate dispensing and removal of the retention device. In various embodiments, when the application/removal device holds the retention device, rotation of the supporting means in a first direction causes the retention device to be dispensed onto the supporting means so as to secure the recording medium thereto, and when the retention device is attached to the supporting means, rotation of the supporting means in a second direction causes the application/removal device to strip the retention device from the supporting means. In preferred embodiments, the application/removal device has a plurality of projections for engaging the interstitial spaces of the retention devices. The application/removal device may also advance toward or away from the supporting means. In a second aspect, the invention comprises a method for securing a recording medium to a rotatable supporting means comprising at least a magnetically susceptible surface. An application/removal device and an elongated retention device having at least one magnetic element is provided.
The foregoing and other objects, features and advantages of the present invention disclosed herein, as well as the invention itself, will be more fully understood from the following description of preferred embodiments and claims, when read together with the accompanying drawings. In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.